The compressor section of a gas turbine engine includes a diffuser downstream of the centrifugal compressor turbines and impeller upstream of the combustor. The function of a diffuser is to reduce the velocity of the compressed air and simultaneously increase the static pressure thereby preparing the air for entry into the combustor at a low velocity. High pressure low velocity air presented to the combustor section is essential for proper fuel mixing and efficient combustion.
The present invention is particularly applicable to gas turbine engines which include a centrifugal impeller as the high pressure stage of the compressor. Impellers are used generally in smaller gas turbine engines. A compressor section may include axial or mixed flow compressor stages with the centrifugal impeller as the high pressure section, or alternatively a low pressure impeller and high pressure impeller may be joined in series.
A centrifugal compressor impeller draws air axially from a low diameter. Rotation of the impeller increases the velocity of the air flow as the input air is directed over impeller vanes to flow in a radially outward direction under centrifugal force. In order to redirect the radial flow of air exiting the impeller to an annular axial flow for presentation to the combustor, a diffuser assembly is provided to redirect the air from radial to axial flow and to reduce the velocity and increase static pressure.
A conventional diffuser assembly generally comprises a machined ring which surrounds the periphery of the impeller for capturing the radial flow of air and redirecting it through generally tangential orifices into an array of diffuser tubes. The diffuser tubes are generally brazed or mechanically connected to the ring and have an increasing cross-section rearwardly. As a result, the narrow stream of air at high pressure taken into the orifices in the ring are expanded in volume as the air travels axially through the diffuser tubes. By the well known Bernoulli theorem (which states that total energy of a fluid flow remains constant being the sum of the pressure energy, potential energy and kinetic energy) the increase in volume results in a reduced velocity and corresponding increase in static pressure.
Fabrication of the diffuser tubes is extremely complex since they have a flared internal pathway that curves from a generally radial tangential direction to an axial rearward direction. Each tube must be manufactured to close tolerances individually and then assembled to the machined central ring. Complex tooling and labour intensive manufacturing procedures result in a relatively high cost for preparation of the diffusers.
In operation as well, diffusers often cause problems resulting from the vibration of the individual diffuser tubes. To remedy vibration difficulties, the diffuser tubes may be joined together or may be balanced during maintenance procedures.
From an aerodynamic standpoint the joining of individual diffuser tubes to the machined ring results in surface transitions which detrimentally effect the efficiency of the engine. On the interior of the tube as it joins the orifice in the ring, there is often a step or transition caused by manufacturing tolerances in the assembly and brazing procedures. Since the air in this section flows at extremely high velocity, the disturbance in air flow and increase in drag as the air flows over inaccurately fit transitions can result in very high losses in efficiency.
In general, the design of diffusers is not optimal since their complex structure requires a compromise between the desired aerodynamic properties and the practical limits of manufacturing procedures. For example, the orifices in the impeller surrounding ring are limited in shape to cylindrical bores or conical bores due to the limits of economical drilling procedures. To provide elliptical holes for example, would involve prohibitively high costs in preparation and quality control. The shape of the diffuser pipes themselves is also limited by the practical considerations of forming their complex geometry. In general, the diffuser tubes are made in a conical shape and bent to their helical final shape prior to brazing. Whether or not this conical configuration is optimal for aerodynamic efficiency becomes secondary to the considerations of economical manufacturing.
It is an aim of the invention therefore, to provide a diffuser assembly which significantly reduces the tooling and manufacturing costs associated with prior art diffuser assemblies.
It is a further aim of the invention to provide a diffuser assembly which provides greater flexibility to the designers of gas turbine engines enabling them to optimize the diffuser structure for improved aerodynamic efficiency and vibration behaviour without concern for the manner in which the diffuser will be actually manufactured.
It is a further aim of the invention to provide a diffuser assembly which has shorter development time for new engines and considerably shorter lead time in normal production by minimizing the operations required for production.
It is a further aim of the invention to eliminate the internal transversal steps between the diffuser tubes and separate internal machined ring of the prior art.
It is a further aim of the invention to lower the weight of engines by reducing the number of parts in a diffuser assembly, and using curved or variable diffuser ducts to reduce the gas generator case diameter.